This invention relates to a method of operating a dual fuel internal combustion engine of the diesel-type and a dual fuel internal combustion engine of the diesel-type operable according to such method.
Dual fuel internal combustion engines of the diesel-type usually comprise a combustion chamber being at least partly delimited by a piston, a first fuel supply for a first fuel, said first fuel supply being located in the combustion chamber or in an inlet port thereof, and a second fuel supply for a second fuel.
A known method to operate internal combustion engines as described above comprises the steps of:                pre-mixing said first fuel in said combustion chamber and/or in said inlet port        compressing the charge containing the first fuel to conditions that allow auto-ignition of the second fuel,        igniting said first fuel in said combustion chamber by injection and auto-ignition of a second fuel to thereby initiate conditions for pre-mixed flame propagation combustion.        
Such a method may generally be described as an operation mode distinguished by pre-mixed flame propagation combustion. For briefness, in the following, such a method will also be referred to as a “flame propagation mode”.
The present invention relates to the field of dual fuel engines of the diesel-type, in particular such that are suitable for heavy load.
One field of application for such engines is in vehicles, such as trucks, lorries and buses. Other fields of application are ships and boats or power generation using internal combustion engines.
For heavy-load engines, diesel oil is conventionally used as the fuel. Accordingly, there is a vast number of diesel engines whose design and function have been optimised for heavy loads. At present, there is however a great interest for enabling heavy-load engines to run on alternative fuels, such as natural gas or biogas.
The present invention relates to such diesel-type engines, but which have been adapted for use with fuels other than diesel. Hence, in this context, what is meant with a diesel-type engine is in a broader meaning that type of engine which would function satisfactory with conventional diesel fuel.
To this end, a number of dual fuel engines have been proposed. For example, it has been suggested to use a natural gas as the main fuel in a diesel-type engine. When natural gas is used as a primary fuel in a diesel-type engine, it has been proposed to use a
combustion process wherein the natural gas is first pre-mixed with air, whereafter a pilot injection of the second fuel is used for triggering ignition. Thereafter, pre-mixed flame propagation combustion takes place in the combustion chamber.
U.S. Pat. No. 4,955,326 describes such a dual fuel engine system wherein the dual fuel engine includes a fuel oil supply and a fuel gas supply. Means connected between the fuel oil supply and the engine injects small amounts of diesel fuel into the engine. The injection of diesel fuel is limited to pilot injection quantities only.
A problem with engines of the above-mentioned type is that they may suffer from problems with knock. This is believed to be due to uncontrolled auto-ignition of the unburned fuel ahead of the flame, a problem that is enhanced in heavy duty engines by large combustion chamber volumes and low engines speed as compared to light duty engines, and by low local flame propagation velocities as compared to engines designed for pre-mixed charge operation, as for instance gasoline-type of engines.
It is desirable to provide an enhanced method for operating a dual fuel combustion engine of the diesel-type, which is suitable for heavy vehicles.
Preferably, the method should diminish the problems with knock as encountered in prior art engines and, as a consequence, improve fuel efficiency and preferably decrease exhaust emissions of unburned hydrocarbons and CO.
Preferably, the method should enable fuel efficiency similar to that of a conventional diesel engine.
Generally, it is desired that the method should enable combustion with low soot and NOx emissions.
In view of the above, it will be understood that there is also a need for a dual fuel internal combustion engine of the diesel-type for carrying out a method as desired above.
In accordance with an aspect of the invention there is provided a method of operating a dual fuel internal combustion engine of the diesel-type, comprising a combustion chamber being at least partly delimited by a piston, a first fuel supply for a first fuel, said first fuel supply being located in or at the combustion chamber and/or in or at an inlet port thereof, and a second fuel supply for a second fuel, the method comprising the steps of:                Pre-mixing said first fuel in said combustion chamber and/or in said inlet port,        compressing the charge containing the first fuel to conditions that allow auto-ignition of the second fuel,        performing a first injection of the second fuel into said combustion chamber to initiate auto-ignition of said second fuel for igniting said first fuel, thereby initiating conditions for pre-mixed flame propagation combustion of the first fuel.        
Further, the method comprises the step of performing at least one subsequent injection, said subsequent injection supplying additional kinetic energy into the combustion process to thereby enhance turbulence intensity and propagation speed of said flame and/or enhance late mixing in the combustion chamber, so a to improve late oxidation during combustion of the remaining fuel.
Hence, in accordance with the invention, at least one subsequent injection may be used to add kinetic energy to the combustion process. The additional kinetic energy provided by the subsequent injection (or subsequent injections) may be used to create additional small-scale turbulence in the region of the flame in the combustion chamber, which in turn enhances the propagation speed of said flame. Accordingly, the flame front will develop faster and reach into the combustion chamber in shorter time than without such additional turbulence, resulting in a more favourable combustion chamber pressure development over time, resulting in turn in higher (fuel) efficiency of the engine and less risk for knock compared with conventional dual fuel combustion processes without this additional turbulence.
Alternatively, or in addition to a subsequent injection to create additional turbulence, the method may include an additional step of performing a subsequent injection (or subsequent injections) for enhancing late mixing in the combustion chamber so as to improve final oxidation during combustion. Such a subsequent injection may be referred to as a “post-injection”. The purpose of the post-injection-type subsequent injection is, as the purpose of the previously described subsequent injections, to add kinetic energy to the combustion process. However, the kinetic energy added by the post-injection will not primarily be utilised as to increase the flame speed. Instead, the kinetic energy from the post-injection is supplied so as to create turbulence in the later stages of the combustion cycle and hence enhance late global and local mixing of gases remaining in the combustion chamber for improving late oxidation during combustion of the remaining fuel.
Advantageously, said first fuel is pre-mixed for instance with air and/or recycled exhaust gas, to form the charge containing the first fuel.
The first injection of the second fuel into the combustion chamber is sufficient to initiate ignition and hence to initiate pre-mixed flame propagation combustion. In contrast, the purpose of the subsequent injection (or subsequent injections) (after said first injection of the second fuel) is to add kinetic energy initiating additional small scale turbulence in the region of the propagating flame or enhancing mixing of gases in the combustion chamber. Accordingly, the subsequent injections need not necessarily add any fuel to the combustion process. Any gaseous or liquid matter, which may transfer kinetic energy via an injection thereof and which does not disturb the combustion process, may be used.
It is preferred to inject a liquid for the subsequent injections, since the necessary quantity of supplied kinetic energy and the administration of the injections may generally be easier to achieve with liquids than with injection of gaseous matter. In view of the above, any liquid which does not disturb the combustion process could be injected, such as e.g. water. It is to be understood however, that with injection of a liquid is meant that the substance is in its liquid phase at the precise moment of injection. Once injected, the liquid will, for typical conditions, transfer to gaseous phase with a rate depending on the combustion chamber conditions, the injection process and the properties of the liquid. Hence, even when the subsequent injections are liquid injections, a large fraction of the kinetic energy is transferred and a large fraction of the turbulence is created by impact of the injected substance while in its gaseous phase.
However, for practical purposes, it is preferred that the subsequent injection is an injection of a fuel. Most preferred, the subsequent liquid injection is of the same fuel as the first injection, that is, of the second fuel. As such, no additional constructive or engine design arrangements for the subsequent injections are necessary.
Preferably, the amount of additional kinetic energy supplied by the at least one subsequent injection is variable.
As mentioned above, it is envisaged to use a combustion cycle comprising at least one subsequent injection, but several subsequent injections may also be used.
Advantageously, the amount of additional kinetic energy supplied by these subsequent injections is variable by varying at least one of: the number of subsequent injections, the injection pressures of the subsequent injections, the durations of the subsequent injections, or the dwell time between subsequent injections. In this context it is to be understood that the amount of additional kinetic energy supplied may be varied between subsequent injections of the same combustion cycle, and/or be varied between subsequent injections belonging to different combustion cycles.
Advantageously, the amount of kinetic energy supplied by the subsequent injection is varied in accordance with feedback supplied from at least one sensor sensing at least one of: the combustion process, the fuels and their parameters as temperature, pressure, time, quality of fuel, etc. and/or the results of said combustion process including but not limited to the resulting chemical compounds produced by such combustion process and contained in the exhaust gas of the engine. The control of the at least one subsequent injection may then be performed using feedback from said sensor as input. Accordingly, the combustion process may be continuously surveyed and the subsequent injections continuously controlled to improve said combustion process. Advantageously, in a startup phase a first combustion cycle or a first few combustion cycles may be run using selected start parameters for the subsequent injection, whereafter the sensor is used for evaluating said first combustion cycle or the first few combustion cycles, and providing input for the adjustment of the subsequent injection for the following combustion cycles in the normal operational phase of the engine. Accordingly, the combustion process may be adapted for a specific situation. Moreover, this enables a more robust combustion process, since the subsequent injections may also be adapted if the conditions should vary during continuous operation of the engine.
As an alternative to initiating the first combustion cycle or the first few combustion cycles with selected start parameters for the subsequent injection, in the start-up phase the first combustion cycle or the first few combustion cycles could be run without any subsequent injection at all.
The sensor(s) may advantageously be sensor(s) for sensing for instance the quality of the supplied fuel, preferably for sensing the octane number. Such sensors could be provided e.g. in the fuel tank or fuel supply system.
Alternatively, in addition or in combination with such a fuel quality sensor, the method may use a sensor for sensing the quality of the combustion. Various such sensors may be applicable. One known type of sensor is a so called knock-sensor. Preferably, a device for sensing the combustion chamber pressure could be used in combination with a method for a fast evaluation (on a cycle-to-cycle basis) of the heat release.
Preferably, the first fuel is a fuel with high octane number, preferably greater than 90. Advantageously, the first fuel is a gaseous fuel, preferably natural gas or biogas.
Preferably, the second fuel is a fuel with high cetane number, preferably greater than 40. Advantageously, said second fuel is a liquid fuel, preferably diesel oil, bio-diesel or DME (dimethyl ether).
Advantageously, said first fuel is pre-mixed with air and/or recycled exhaust gas.
In a second aspect of the invention, there is provided a dual fuel internal combustion engine of the diesel-type comprising:                a combustion chamber being at least partly delimited by a piston,        a first fuel supply for a first fuel, said first fuel supply being located in or at the combustion chamber and/or in or at an inlet port thereof,        a second fuel supply for a second fuel,        said first fuel being pre-mixed in said combustion chamber and/or said inlet port,        at least one device for compressing the charge containing the first fuel to conditions that allow auto-ignition of the second fuel,        at least one injector device for performing a first injection of the second fuel into said combustion chamber to initiate auto-ignition of said second fuel for igniting said first fuel, thereby initiating conditions for pre-mixed flame propagation combustion of the first fuel, wherein        
at least one injector device for performing at least one subsequent, preferably liquid, injection, so as to supply additional kinetic energy into the combustion process to thereby enhance turbulence intensity and propagation speed of said flame and/or enhance late mixing in the combustion chamber so as to improve late oxidation during combustion of the remaining fuel.
Preferably, the dual fuel internal combustion engine comprises at least one sensor for providing feedback from the combustion process, wherein the output from said sensor is used for said control of the at least one subsequent, preferably liquid, injection.
Preferably, the dual fuel internal combustion engine comprises a sensor for sensing the quality of the combustion, or the quality of the supplied fuel.
It should be realised that alternatives and advantages as described above in relation to the method applies equally to the dual fuel internal combustion engine according to the invention.